The present invention relates to a tachometer for measuring the rotating speed of an engine or the speed of a vehicle which is detected in the form of a rotating speed and, more particularly, to a tachometer which employs a cross-coil type indicator in its indicating portion.
Tachometers heretofore employed for vehicles are the moving-coil type. The moving-coil type tachometer is what is called a precision instrument, which is complex in construction, easy to break and large in the number of parts used and calls for many troublesome steps for the assembling thereof, and hence is expensive.
In view of the above, the cross-coil type tachometer has recently come into use. The cross-coil type tachometer is disclosed, for example, in U.S. Pat. No. 3,327,208 issued June 20, 1967. The cross-coil type tachometer is simple in construction, small in the number of parts used, easy to assemble, inexpensive and, in addition, sturdy and highly reliable. With the cross-coil type indicator, when cutting off the supply of its power source voltage, the pointer usually remains at the position of indication immediately before the voltage supply is cut off. In the case where the cross-coil type indicator is used in a tachometer for detecting the engine speed of a vehicle, it is not preferable that even after an ignition switch is turned off to stop the rotation of the engine, the indicator still indicates the engine speed immediately before the ignition switch is turned off. To avoid this, the tachometer is equipped with zero resetting means for resetting the pointer to a reference position (a zero point) when the supply of the working voltage is cut off. The zero resetting means for use in the moving-coil type indicator employs a hair spring but this introduces complexity in the indicator structure, offsetting the advantages of the cross-coil type indicator.
There has been proposed zero resetting means of the type in which a small zero resetting permanent magnet is provided adjacent a pointer driving magnet. In this case, if the magnetic force of the zero resetting magnet is increased, no correct indications are provided under the influence of the increased magnetic force and, in addition, errors differ with the angular positions of the pointer. The torque for returning the pointer to the reference position by the zero resetting magnet differs with the angular positions of indications by the pointer, so that a decrease in the magnetic force of the zero resetting magnet introduces the possibility that the pointer cannot be returned to the reference position depending on the angular position of the pointer at the time of the power source being cut off. In order that the zero resetting magnet may have a sufficient torque for returning the pointer to the reference position, and that the influence of the zero resetting magnet on the indication may be made negligible, it is necessary that the intensity of the magnetic field of the zero resetting magnet be below 1% of the signal magnetic field by the cross coil for energizing the pointer driving magnet. To perform this, the cross coil must be supplied with a large drive current and given a large number of turns; therefore, an increase in the magnetic force of the zero resetting magnet is not preferable.
In a cross-coil type indicator adapted for providing graduations over a wide angular range of more than 180 degrees, it may happen that, depending on the angular position of the pointer at the time of turning off the power supply, the pointer, when returning to the reference position, is turned by the zero resetting magnet in the direction in which the meter indication increases. This causes such an impression as if the engine or the indicator is out of order. Furthermore, when a stopper is used for stopping the pointer at the reference position, the pointer cannot be correctly brought back to the reference position in the abovesaid case and, when driven next, the pointer does not turn in the direction in which the meter indication increases, but it turns in the opposite direction to indicate the rotating speed at that time; this also produces a strange feeling on the user.